We requested this some time ago as well, but in a different way...
When a custom form is selected, autofill the default form with some predefined stuff and make them readonly/ hide them or allow the user to change the predefined / prefilled default form.
Here's the link to the old thread: http://forum.osticket.com/d/discussion//
Maybe in a future release it's possible, but that's osTicket developers / Peter's & Jared's decision.